I think many people think that because somebody has an erotic fantasy about something, they at least partly want to do it in reality. With something like corporal punishment that isn't such a big deal, but incest and murder and paedophilia are pretty serious, deeply held taboos - and while there's no really consistent logical reason for incest to be forbidden by society, people are indoctrinated to find it really disgusting. And murder and padeophilia are taboo for pretty good and obvious reasons. So it creeps people out a bit to see that they turn others on.Originally Posted by Timberwolf
None of which makes it sensible, but that's at least part of the reason behind it IMHO.